This invention relates generally to a resonant microcavity and, more particularly, to a resonant microcavity which produces a strong light-matter interaction.
A resonant microcavity, typically, is formed in a substrate, such as silicon, and has dimensions that are on the order of microns or fractions of microns. The resonant microcavity contains optically-active matter (i.e., luminescent material) and reflectors which confine light in the optically-active matter. The confined light interacts with the optically-active matter to produce a light-matter interaction.
The light-matter interaction in a microcavity can be characterized as strong or weak. Weak interactions do not alter energy levels in the matter, whereas strong interactions alter energy levels in the matter. In strong light-matter interaction arrangements, the confined light can be made to resonate with these energy level transitions to change properties of the microcavity.
In general, in one aspect, the invention is a structure, i.e., a condensed matter system, including a resonant microcavity formed by reflectors (with a reflectivity R) arranged relative to an optically-active material to form a cavity. The optically-active material has a thickness L, an optical emission line centered at a wavelength xcexc, and an optical absorption coefficient xcex10 at xcexc. The optical emission line has a homogeneous spectral width xcex4 and an inhomogeneous spectral width xcex94. In the structure, xcex10L greater than  greater than (1xe2x88x92R) and   Δ   less than       2    ⁢                                                      α              0                        ⁢            L            ⁢                          xe2x80x83                        ⁢                          δ              2                                            (                          1              -              R                        )                              .      
As a result, the microcavity produces strong light-matter interactions between the optically-active material and an electromagnetic field confined in the cavity.
Strong light-matter interactions mix the states of the optically-active material and confined light (vacuum electromagnetic field) in the microcavity to produce two new mixed states, each of which are a superposition of the optically-active material state and the vacuum electromagnetic field state. The two new mixed states are split apart in energy by the so-called Vacuum Rabi Splitting (VRS). The VRS is on the order of a few meV (nanometers). The energy splitting between the two non-degenerate mixed states is a function of the reflectivity (R) of the microcavity, the thickness (L) and the absorption strength (xcex10) of the optically-active material. Altering one or more of these parameters changes the energy by which the two mixed states are split which, in turn, affects the reflectivity characteristics of the microcavity at the wavelength of the two states. For a specific set of VRS parameters, R, L, and xcex10, the reflectivity characteristics of the two mixed states also can be changed by xe2x80x9ctuningxe2x80x9d the length of the microcavity to alter the resonant cavity mode such that it is in or out of resonance with the optically-active material emission line. For example, tuned and untuned microcavities may transmit/reflect different wavelengths of light and can be used to construct tunable optical devices which switch between reflecting and transmitting certain wavelengths.
The intensity (population) in the two superposition states oscillates back and forth at a specific rate commonly called the Rabi oscillation frequency. The Rabi oscillation frequency for the two non-degenerate mixed states in the microcavity can be in the gigahertz (GHz) to terahertz (THz) range. As a result, the microcavity can be used in THz devices, such as emitters and detectors.
This aspect of the invention may include one or more of the following features/functions. The ratio xcex10L/(1xe2x88x92R) is greater than about 10; greater than about 100; or greater than about 10006. The ratio   (            Δ      /      2        ⁢                                        α            0                    ⁢          L          ⁢                      xe2x80x83                    ⁢                      δ            2                                    (                      1            -            R                    )                      )
is less than about 1; less than about 0.1; or less than about 0.01. The microcavity may be a Fabry-Perot cavity having an optical resonance at wavelengths between about xcexxe2x88x922xcex94 and about xcex+2xcex94; between about xcexxe2x88x92xcex94 and about xcex+xcex94; or between about xcexxe2x88x920.1xcex94 and about xcex+0.1xcex94. The reflectors may be arranged at opposite sides of the optically-active material.
The reflectors may include metal films and/or Distributed Bragg Reflectors (DBR). The DBRs may be arranged on opposite sides of the microcavity. Each DBR typically includes alternating layers of material having different indices of refraction. The alternating layers of material may include one or more of the following: semiconductors, conductive matter oxides, glasses, glass-like oxides, and polymers. The alternating layers typically have high and low indices of refraction nH and nL and thicknesses on the order of xcexc/4nH and xcexc/4nL, respectively. Each DBR may include between 2 and 22 alternating layers. The thickness of the microcavity is an integer multiple of a half wavelength, xcexc/2, divided by the index of refraction of the optically-active matter, n.
The optically-active material in the microcavity may include Er or Er2O3 embedded in a layer of SiO2, crystalline Er2O3, a rare earth doped layer of Si, and/or a rare earth doped layer of SiO2. The material may include a rare earth doped host matter such as one or more of the lanthanide series elements with numbers 57 through 71. The host matter has a negligible luminescence at xcex. The material may include a semiconducting light-emitting matter, such as one or more of the following: GaAs, GaAlAs, GaInAs, GaInPAs, GaN, GaP, and InP. The material may include a doped semiconductor which emits radiation at a wavelength that is higher than a semiconductor band gap transition associated with the doped semiconductor.
The structure may be included in an apparatus, such as an optical switch, a THz emitter, a THz detector, a wavelength modulator, a wavelength-division multiplexer, and/or a waveguide cross-connect.
The foregoing condensed matter structure has the following advantages. The structure does not require inordinately high finesse or Q values to achieve strong light-matter interactions. The resonant microcavity is operable over a relatively wide range of temperatures, including room temperature. The strong light-matter interaction also provides a sensitive cavity structure, i.e., a resonator, which responds to changes in the index of refraction of the reflectors. Additionally, the resonator can be produced with a wide range of optically-active materials. Consequently, the structure has a wide range of applications in electronic and optoelectronic devices.
Other advantages and features will become apparent from the following description, claims, and drawings.